To improve productivity, an apparatus for fabricating semiconductor devices employs a system in which a plurality of processing chambers are connected to a single core chamber, namely a cluster-type system.
A core chamber is generally a regular polygonal structure, the faces of which are connected to processing chambers. A transfer robot is installed in the core chamber to transfer wafers. The transfer robot is arranged on the axis of the core chamber to transfer wafers to each one of the processing chambers. The transfer robot includes a hand, which supports a wafer. The hand is moved in a circumferential direction (θ direction) of a circumscribing circle of the core chamber and a radial direction (R direction). A wafer accommodated in a load lock chamber is supported by the hand of the transfer robot and transferred into a processing chamber by an R-θ plane action of the hand.
In a cluster system, to obtain high reproducibility for the processing state of each processing chamber, it is important that wafers be accurately transferred to each processing chamber. The transfer robot is also required to detect the position of a wafer during a wafer transfer process and correct the transfer state of the wafer based on the detection result. For an apparatus that fabricates semiconductor devices, detection of the center of the wafer during the transfer process has been proposed in the prior art to improve the transfer accuracy of the wafer.
Patent document 1 discloses the arrangement of a sensor array, which includes a plurality of sensors, for example, three sensors, in a core chamber. Each sensor is set along an axis that traverses a movement route of a wafer in the R direction. Each sensor in the sensor array detects an edge surface of a wafer as a sensor trigger point and generates an output signal indicating the position of the wafer in relation with the sensor trigger point. The transfer robot calculates the location of a wafer center relative to a target point in response to three output signals from the sensor array so as to align the wafer center with a target point. In this manner, the position of the wafer is detected during a loading-unloading operation of the wafer. Thus, position information of the wafer is accurately obtained without stopping the movement of the transfer robot.
In patent document 2, each sensor of a sensor array is arranged along an axis that transverses a movement route of a wafer in the θ direction. Each sensor in the sensor array detects an edge surface of a wafer as a sensor trigger point and generates an output signal indicating the position of the wafer in relation with the sensor trigger point. As a result, the sensor array may detect the edge surface of a wafer without the need for separately performing a loading-unloading operation of the wafer. This obtains position information in an easier manner.
Patent documents 1 and 2 both require a plurality of sensors arranged along an axis traversing the movement route of a wafer. This is because during detection of an edge surface of the wafer, the wafer serving as the detection subject is moved in only one direction (the R direction or the θ direction). Further, a single sensor and two detection points on the edge surface detected by the sensor must all be located along a straight line in the movement direction of the wafer.
Specifically, in patent document 1, the sensor and the two detection points detected by the sensor are arranged along a straight line in the R direction. Thus, to obtain the required detection accuracy, a different detection point must also be set in at least a direction traversing the R direction. Accordingly, a plurality of sensors are necessary when moving a wafer in the R direction. In patent document 2, the sensor and the two detection points detected by the sensor are arranged in the θ direction. However, the pivoting radius of the hand is much greater than the substrate radius. Thus, the sensor and the corresponding two detection points are substantially located on a generally straight line when entirely viewing a substrate.
Accordingly, in patent documents 1 and 2, when correcting the position of the wafer for each processing chamber, the quantity of the sensors must be in correspondence with the quantity of the processing chambers. For example, when three sensors are required for a single processing chamber, twenty-four sensors are required for a cluster system including eight processing chambers.
As a result, patent documents 1 and 2 greatly increase the number of components in the fabrication apparatus. This causes the apparatus structure to be complicated and increases costs.    Patent Document 1: Japanese Examined Patent Publication No. 7-27953    Patent Document 2: Japanese Laid-Open Patent Publication No. 6-224284